Many mechanical devices may adopt one of two substantially fixed configurations and are repeatedly cycled between two operating positions. These positions may be designated as off/on or open/closed or extended/retracted or engaged/disengaged or some similar terminology. Exemplary automotive examples of such devices include a retractable air dam, latches and clutches among others. Other mechanical devices may adopt, in addition to ‘open’ and ‘closed’ positions a range of positions intermediate between these limits. Exemplary automotive examples of such devices are a set of louvers for controlled passage of air which may be ‘closed’, ‘open’ 50% open, 85% open etc., a fluid flow valve, rearview or side mirrors or visors or shades. Both types of devices may be operated using mechanisms incorporating an elongated shape memory alloy (SMA) actuator.
SMA actuated devices find particular application in vehicles where their low mass coupled with their reliability and relative simplicity makes them attractive replacements for electromechanical devices such as electric solenoids or motors. This is particularly so when the stroke or range of operation of the device is limited.
SMA actuators rely for their operation on the useful property of SMA alloys that they may forcibly shrink or shorten in length when heated. The force generated by such SMAs is significant and may be powerful enough to operate a device even when some mechanical or other resistance is encountered.
A device may incorporate various components and mechanisms to achieve a desired range of motions but the key components of the SMA actuator are a preselected length of SMA alloy arranged in series with a biasing element which creates a biasing force to reset the device and prepares it for re-use. A common form of a biasing element is a spring but other approaches such as dead weights or hydraulic cylinders, among others, may be used. The SMA may suitably be in the form of wire, tape, chain, cable, braid or any other elongated form of SMA capable of sustaining a tensile load. The spring is commonly attached at one end to a support and at its other end to one end of the SMA wire, with the second end of the SMA wire attached to a second support. The workpiece or component to be moved is positioned between the wire and the spring. Mechanical stops may be employed to enforce only an intended range of motion.
SMAs derive their useful properties from their ability to exist in two crystalline phases, a first, lower modulus, phase stable at lower temperatures, and a higher modulus, higher temperature phase of a different crystal structure. The transition from one phase to the other may, by appropriate choice of alloy system, alloy composition, heat treatment or applied stress, be selected to occur over a temperature span of from −100° C. up to about +150° C. or so. But, many useful SMA alloys exist in their lower temperature or martensite phase, at, or slightly above, about 25° C. or so, and transform to their higher temperature, or austenite, phase at temperatures ranging from about 60°-80° C. or so. These characteristics substantially assure that the SMA will be in its martensite phase at ambient temperature but may be readily transformed to its austenite phase with only modest heating.
SMA actuator wires, or similar, are first shaped, in their austenite phase to the desired form, then cooled to ambient temperature, resulting in their adopting the martensite crystal structure. While in their martensite phase the wire is stretched and deformed to its intended predetermined length. The deformation exceeds the maximum allowable elastic strain which may be imposed on the actuator, and is often termed pseudo-plastic deformation. These pseudo-plastically-deformed martensite wires are in the appropriate starting condition for the actuator.
Generally the stretch or strain, that is, the change in length of the wire divided by its original or base length, applied during such pseudo-plastic deformation does not exceed 7% and more commonly may be 5% or less. Importantly, the base length, to which all length changes are referred, is the length of the wire in its high temperature, austenite phase.
After being suitably deformed in their martensite phase, the SMA wires may, when heated and transformed to austenite, spontaneously revert to their original undeformed shape. In changing shape, the wire will contract by an amount substantially equal to the pseudo-plastic strain previously applied when it was in its martensite phase. So, by suitable choice of wire length, any desired displacement may be achieved. As an example, a 10 inch or so length of wire, prestrained to about 5% strain, may enable a total displacement of about one-half inch or so. The force applied during heating may be increased by increasing the wire diameter, or, more commonly, to facilitate prompt cooling of the device, by arranging multiple smaller diameter wires in parallel.
Actuator action may be reversed by stopping heating and allowing the wire to cool to about ambient temperature and revert to its martensite crystal structure. During cooling the SMA wire will not spontaneously change its length to its initial deformed length but, in its martensite phase, it may be readily stretched again to its initial predetermined length. The spring, or other biasing element, in series with the SMA wire is selected to deform the SMA when the SMA is in its less strong martensite phase. So, on cooling, when the austenite wire reverts to its martensite phase it is stretched, by the spring, to its initial length so that the cycle may be repeated. Provided the transition in crystal structure is fully reversible this cycle of extending and contracting the wire by application of suitable stimulus may continue indefinitely.
In practice however, the phase transitions and the accompanying cyclic length are not completely reversible, and some irrecoverable deformation occurs. These cycle-by-cycle irreversibilities accumulate over repeated cycles to permanently extend the wire. This permanent extension of the wire introduces slack into the initially-taut wire and both reduces the stroke obtainable from a device and renders its operation non-linear. These effects may be sufficient to impair the operation of the device or render it ineffective. In such a circumstance the device may need to be replaced.
There is therefore a need to mitigate the effects of irreversible transformation of SMA actuators in devices to extend the useful cycle life of such devices.